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Electric potential difference and Ohm's law review

Review the key terms, equations, and skills related to Ohm's law, including how electric potential difference, current, and resistance are related.

Pojęcia kluczowe

BatteryDevice that transforms chemical energy into electrical energy. An ideal battery has no internal resistance.
Electric potential difference (ΔV)Energy change per unit charge between two points. Also called voltage or electric potential. Has SI units of Volts V=JC.
Electromotive force (EMF, ϵ)EMF is the potential difference produced by a source such as an ideal battery. Has SI units of V.


EquationSymbolsMeaning in words
I=ΔVRI is current, ΔV is electric potential difference, and R is resistanceCurrent is directly proportional to electric potential difference and inversely proportional to resistance.

Ohm’s Law

Ohm’s law states that for some devices there is a relationship between electric potential difference, current, and resistance.
The equation is: I=ΔVR
Where I is current, ΔV is electric potential difference, and R is resistance.

How are electric potential difference and current related?

For a given resistance R, increasing the electric potential difference ΔV increases the current I and vice versa.

How are current and resistance related?

For a given electric potential difference ΔV, if the resistance R increases, then the current I decreases and vice versa.

How are resistance and electric potential difference related?

For a given current I, if the electric potential difference ΔV increases, then the resistance R also increases and vice versa.

Analyzing electric potential difference across a resistor using Ohm’s law

If the current encounters resistance, the electric potential difference decreases according to Ohm’s law. We sometimes call this a voltage drop.
Figure 1: Electric potential drop across a resistor

Analyzing electric potential difference and current across a battery

A common source of electric potential is a battery, which is represented in diagrams by the symbol below (Figure 2). The short side is the negative end, with a lower electric potential, and the long side is the positive end, with a higher electric potential.
Electrons flow from the negative terminal to the positive terminal. Conventional current I travels from the positive terminal (higher electric potential), through the circuit, and finally to the negative end (lower electric potential).
Figure 2: The symbol for a battery. The long side is the positive terminal, and the short side is the negative terminal.
Current flow and electric potential difference can be better understood by using the analogy of a boulder rolling down a hill. At the top of the hill, the boulder has a lot of gravitational potential energy. Similarly, an electron has a lot of stored energy in the form of electric potential energy when it is at the negative terminal of a battery. The boulder will naturally fall toward the ground where potential energy is lower. The electron at the negative terminal of a battery will naturally flow toward the positive terminal, where the electric potential is lower.
As the boulder falls downward, the stored energy is converted to kinetic energy. As the electron flows across electrical components, the stored energy is converted into various forms of energy such as heat and light.

Często spotykane błędy i nieporozumienia

Sometimes people think all devices follow Ohm’s law. However, a device is only ohmic when the current is directly proportional to the electric potential difference, and inversely proportional to the resistance. If we plotted an electric potential vs. current graph for an ohmic device, the relationship would be linear (see Figure 3).
Figure 3: Relationship between electric potential difference and current for ohmic and non-ohmic devices.
Some devices such as light bulbs are non-ohmic. This means that their electric potential difference-current graphs are non-linear, as in Figure 3. For non-ohmic devices, we can’t use I=ΔVR to solve for an unknown.

Dowiedz się więcej

For deeper explanations on electric potential and Ohm's law, see our video on circuits and Ohm's law.
To check your understanding and work toward mastering these concepts, check out the exercise on Ohm's law.

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